This invention relates to optical networks that use wavelength division multiplexing (WDM), and, more specifically, to internetworking of such networks with connectionless (CL) packet networks such as internet protocol (IP) networks.
Known optical networks use wavelength division multiplexing (WDM) for point-to-point communication between nodes disposed on an optical transmission medium such as an optical fiber. Data, i.e. information bearing packets associated with a plurality of individual calls, is used to modulate a laser having a wavelength corresponding to a particular WDM channel, and the optical signal is inserted onto the transmission medium in an optical add/drop module (OADM) at one node. The optical signal is transported on the medium to a destination node, where another OADM extracts the optical signal, whereupon demodulation is performed to recover the data.
An arrangement for internetworking of optical and connectionless packet networks such as Internet Protocol (IP) networks, is described in a copending application Ser. No. 09/333406 and filed Jun. 15, 1999 and entitled xe2x80x9cWideband Optical Packet Ring Networkxe2x80x9d, assigned to the same assignee as the present invention, which is incorporated herein by reference. In the aforementioned application, specially equipped routers in the packet networks are arranged to have optical interfaces to OADM""s in the optical network. These routers, sometimes referred to as xe2x80x9coptical gatewaysxe2x80x9d or simply xe2x80x9cgatewaysxe2x80x9d, include hardware and software that performs several functions. First, each gateway includes a packet framer and an optical transceiver that converts an xe2x80x9celectricalxe2x80x9d stream of packets into an optical signal that modules a laser having a particular wavelength. Second, each gateway is functionally arranged to control the OADM""s in order to implement routing tables that associate specific destinations on the optical network (i.e., remote gateways connected to other OADM""s) with a particular xe2x80x9cportxe2x80x9d on the router. The laser output, which is available at the above-mentioned particular port on the gateway, is then combined with (added to) other wavelengths on the optical transmission medium to form the WDM signal. At the destination gateway, the portion of the optical signal at the particular wavelength is then extracted (dropped) from the other wavelengths on the optical transmission medium, and the optical signal is reconverted to a stream of packets in another optical transceiver, which can then be transmitted on toward a desired destination. The path which an IP packet takes through WDM network is determined by the wavelength on which it enters the WDM network and the state of the particular ones of the OADMs through which the packet travels.
A prior art arrangement of the type just described is illustrated in FIG. 1. A packet network 110 includes a plurality of interconnected routers, such as conventional routers 111 and 112, and gateways 121 and 122, which are routers that interface both with conventional routers and also with particular OADM""s in an optical WDM network designated generally at 150. Thus, gateway 121 has a connection to OADM 151, while gateway 122 has a connection to OADM 152. An optical WDM transmission medium 160 with counter-clockwise optical flow, interconnects OADM 151 to OADM 153, OADM 153 to OADM 154, and OADM 154 to OADM 152. OADMs 153 and 154 are, in turn, connected to gateways 133 and 134, respectively, which may be part of packet network 110 or may be part of a different packet network.
For simplicity of description, assume that gateways 121, 122, 133 and 134 each have two ports, called port 1 and port 2, each arranged as part of an electrical to optical interface at a specific wavelength xcex1 and xcex2, respectively. In a real implementation (such as an implementation using AllWave(trademark) fiber technology available from Lucent Technologies), each gateway could be arranged to simultaneously support many more WDM channels; (e.g., as many as 2000 channels) on transmission medium 160. Each router in packet network 110, including gateways 121, 122, 133, 134 and 135, has a routing table, which specifies which port an incoming packet (i.e., a packet received from another router in the packet network) should be applied to in order to be transported on the optical network to a particular destination gateway. Thus, for example, as shown in FIG. 1, the routing tables for gateways 121 and 134 may be as set forth in tables 1 and 2 below, respectively:
In the example of FIG. 1, assume that a first xe2x80x9cconnectionxe2x80x9d from gateway 121 to gateway 133 is desired. This connection can be established through the optical WDM network by applying packets received at gateway 121 (from other routers in the packet network 110) to port 1, which is associated in Table 1 with the desired destination (gateway 133). The packets are used to modulate a laser having a wavelength xcex1 associated with port 1, and are inserted via OADM 151 onto transmission medium 160, which in this case is arranged to xe2x80x9caddxe2x80x9d the laser output to the signals already travelling in a counterclockwise direction on transmission medium 160. In this example, OADM 153 is arranged to extract (drop) the optical signal on transmission medium 160 at wavelength xcex1 from the other *WDM signals on the transmission medium, so that the information bearing packets can be recovered by demodulation in a transceiver in gateway 133. Also assume that a second connection from gateway 134 to gateway 122 is desired. This connection can be established through the optical WDM network by applying packets received at gateway 134 (from other routers in another packet network not shown in FIG. 1) to port 2, which is -associated in Table 2 with the desired destination (gateway 122). The packets are used to modulate a laser having wavelength xcex2 associated with port 2, and are inserted via OADM 154 onto transmission medium 160, which in this case is arranged to xe2x80x9caddxe2x80x9d the laser output to the signals already travelling in a counterclockwise direction on transmission medium 160. In this example, OADM 152 is arranged to extract (drop) the optical signal on transmission medium 160 at wavelength xcex2 from the other WDM signals on the transmission medium, so that the information bearing packets can be recovered by demodulation in a transceiver in gateway 122.
While the first and second connections just described are ongoing, it will be observed that if a connection from gateway 121 to gateway 122 is concurrently requested, the request would have to be denied. This would be true even though gateway 121 has an idle port, namely port 2. This is because if port 2 were to be used, the incoming packets would be used to modulate a laser at wavelength xcex2. This signal would be added at OADM 151 and dropped at OADM 152. While no interference would occur in the portion of transmission medium 160 between OADM 151 and OADM 154, it is noted that the same wavelength, xcex2, would be used in the portion of transmission medium 160 between OADM 154 and OADM 152, causing impermissible interference. Accordingly, the general object of the present invention is to enable efficient allocation of network resources (e.g., bandwidth) in an optical WDM network, and provide the ability to internetwork optical and packet networks so as to provide truly guaranteed connections to satisfy service requirements. A specific object is to enable intermetworking of optical WDM and packet networks in a manner in which the previously described interference is avoided.
In accordance with the present invention, a connectionless packet network and an optical WDM network are interconnected by one or more optical interface modules (gateways) that include both optical to electrical interfaces, as well as a connection management module, or control element, that is arranged to control the OADM""s and the configuration of lasers and port assignments within the gateways, such that a route through the optical network to a desired endpoint is selected.
In one embodiment of the present invention, the OADM""s can be programmed, (i.e., locally or remotely controlled by the control element) such that the wavelengths that can be added or dropped by an OADM can be changed. This flexibility allows routes to be established through the optical network, from an originating gateway to a destination gateway, under circumstances such as those described above in conjunction with FIG. 1, where a route would otherwise be unavailable. In this embodiment, changes to the routing tables of at least some of the gateways are generally required. The programmable OADM""s can use fiber Bragg grating technology in which the gratings are tuned using temperature or magnetic-strain. Alternatively, thin-film technology can be used in which tuning is realized by mechanically translating the filter.
In another embodiment of the present invention, the OADM""s are programmable, and, in addition, the optical interface modules include a plurality of N tunable lasers, where N is an integer equal to the number of WDM channels present on the optical transmission medium. With this capability to change the wavelength of the laser associated with a particular port in the gateway, routes can be established through the optical network, from an originating gateway to a destination gateway, under circumstances such as those described above in conjunction with FIG. 1, where a route would otherwise be unavailable, and, in addition, changes to the routing table that associates particular endpoints with particular ports is advantageously not required. If the system is operated in a xe2x80x9cprovisionedxe2x80x9d mode, where connections are set up a priori (i.e., before actual traffic flow starts), based upon resource requirements that are computed using some estimate of the expected traffic, the lasers can be re-tuned on a fairly infrequent basis On the other hand, when the system is operated in a xe2x80x9cswitchedxe2x80x9d mode, where connections are set up on a session by session (call by call) basis, the lasers can be re-tuned much more frequently. In this embodiment, transceiver within the interface modules are arranged to insert xe2x80x9cfillersxe2x80x9d into the optical outputs when no packet data is being inserted onto the WDM channels. This advantageously avoids the need for burst-mode receivers in the interface modules.
In still another embodiment of the present invention, the number of tunable lasers can be fewer than the number of WDM channels available on the optical transmission medium, in which case burst-mode receivers are required in the interface modules. Advantageously, in this arrangement, as in the one just described, the port assignments (routing table) in the gateways do not have to be changed.
The interface modules contemplated by the present invention can be physically integrated with the components otherwise present in conventional packet routers. Alternatively, these modules can be housed separately in intelligent gateways that interconnect conventional packet routers with OADM""s on an optical WDM network. The WDM network can use dense wavelength division multiplexing, or, if desired, coarse WDM of the type described in an article entitled xe2x80x9cOptical Networkingxe2x80x9d, by Daniel Y. Al-Salameh et al., published in the Bell Labs Technical Journal, January-March 1998, pps. 39-61.
The connection management module is arranged to determine, in response to receipt of a stream of packets intended to be routed to a remote destination, the xe2x80x9cshortest pathxe2x80x9d to that destination, which may be through the packet network or through the optical WDM network. If routing through to the optical network is preferred, the management module is arranged to (a) analyze the existing traffic on the optical network to determine if a route is available without changing existing light paths, and (b) if a route is available, to xe2x80x9cset upxe2x80x9d that route, and (c) if a route is not available, to appropriately control various elements in the optical network so as to both shift existing traffic to alternate routes as well as to set up the desired route. The xe2x80x9cset upxe2x80x9d just described may involve tuning the wavelength of a tunable laser, rearranging the port assignment of a fixed laser, and/or reconfiguring of the programmable OADM""s. In some cases, the management module also needs to change routing tables in the optical interface modules.